Waves at an interface

1. An interface is the boundary between two materials, such as air and glass, water and the seabed, or air and a brick wall.
2. When a wave reaches an interface, its energy can be reflected, transmitted or absorbed.
3. In reflection, the wave bounces back into the material it came from.
4. In transmission, the wave passes through the interface and carries on into the second material.
5. In absorption, energy from the wave is transferred to the material, usually increasing its thermal energy store so it warms up slightly.
6. At most interfaces all three happen at once, with the wave's energy shared between them.
7. A window transmits most of the light that reaches it, reflects a little so you can see a faint reflection, and absorbs a small amount.
8. How the energy is shared depends on the two materials, on the wavelength of the wave and on the angle at which it arrives.
9. Energy is always conserved, so the energy reflected, transmitted and absorbed adds up to the energy that arrived.

Reflection and echoes

1. Reflection is strongest when a wave meets a surface it cannot easily pass into, such as light hitting a mirror or sound hitting a hard wall.
2. To describe a reflection, you draw a normal, an imaginary line at right angles to the surface where the wave strikes it.
3. The angle of incidence is measured between the incoming ray and the normal.
4. The angle of reflection is measured between the reflected ray and the normal.
5. For a smooth, flat surface, the law of reflection states that the angle of incidence is equal to the angle of reflection.
6. A reflected sound wave is called an echo, and you hear it as a separate sound if it returns more than about $0.1s$ after the original.
7. Hard, smooth surfaces such as tiled walls reflect sound well, which is why a school sports hall or a swimming pool sounds so echoey.
8. Soft, uneven materials such as curtains, carpets and foam panels absorb much more of the sound, so recording studios and cinemas line their walls with them.

Ultrasound and sonar

1. Ultrasound is sound with a frequency above $20000Hz$, which is too high for humans to hear.
2. When ultrasound reaches the boundary between two different materials, part of it is reflected and the rest is transmitted onwards.
3. Each boundary the ultrasound reaches sends back a weaker echo, so a single pulse produces a series of echoes from boundaries at different depths.
4. Timing each echo shows how far away each boundary is, which allows structures that cannot be seen directly, such as an unborn baby, to be mapped.
5. Sonar uses the same idea under water, with a ship sending a pulse of sound or ultrasound down to the seabed and timing how long the echo takes to return.
6. The pulse travels down to the seabed and back up again, so the depth is half of the total distance the pulse travels.
7. This gives $depth=\frac{speed\times time}{2}$.
8. Fishing boats use sonar to find shoals of fish, and survey ships use it to map the shape of the seabed and to find shipwrecks.

Refraction at an interface

1. When a transmitted wave crosses an interface at an angle, it can change direction, and this is called refraction.
2. Refraction happens because the wave changes speed as it moves into the new material.
3. Light slows down as it passes from air into glass and bends towards the normal, then speeds up as it leaves and bends away from the normal.
4. A wave that meets the interface along the normal changes speed but does not change direction.